Optical Coherence Tomography (OCT) is a method for non-invasive acquisition of high-resolution images of subsurface tissue structure and function. Through the use of autocorrelation phase sensitive detection and Kasai velocity estimator Doppler frequency shifts can currently be estimated in real-time with Doppler OCT systems (DOCT) in real-time. The maximum detectable velocity is determined by the wavelength of the light, refractive index of the medium, Doppler angle, and the time between samples or a-scan frequency, which is detailed later. Given a fixed wavelength, set angle for in vivo applications, and a defined refraction index for the tissue; the sample time is the only dynamic variable. Increasing the a-scan rate is a technological challenge that is continuously being worked on, however, the sampling rate in the axial direction is orders of magnitude higher than fa.
The transverse Kasai (TK) autocorrelation estimator is suitable for imaging slow bi-directional flows representative of microcirculation and is the one currently used on DOCT systems. Aliasing due to the axial scan (a-scan) frequency, however, limits the maximum TK detected non-aliased axial flow speed to <4 mm/s on time-domain OCT systems where rapid scanning optical delay (RSOD) lines operate at scan rates between 8 to 15 kHz. The upper limit can be increased to ˜8 cm/s on spectral domain or swept-source OCT systems with higher effective a-scan rates.
Phase-unwrapping techniques can extend the velocity detection range; however, at high flow rates, separation between aliasing rings can become smaller than the spatial resolution of the imaging system, making phase-unwrapping unreliable. DSP-based autocorrelation with time delays less than the a-scan period and Hilbert transform techniques can provide-higher aliasing limits up to ˜35 cm/s with reduced sensitivity to low flow speed. However, in applications such as coronary imaging, flow velocity estimation in the range of meters per second is required. In addition, blood flow velocity in the microvasculature of atheroma can be orders of magnitude lower than that in the lumen and both velocities can be present in the OCT field of view.
Therefore, there is a need to provide a method that addresses the above-mentioned problems. We have determined a new method for flow estimation that has been in vivo tested to flow velocities of over 1 m/s. Furthermore, because this method does not require additional hardware, it can be adapted onto older systems through a software update and give a resolvable velocity range from the um/s range to the m/s range spanning both microvasculature imaging and cardiac imaging.